This makes it so you can check if the initial connection
is established. Before this we used reconnect which would
lazily attempt to connect. So if you were trying to connect
to a non existant Server you wouldn't find out until after
you attempted your first RPC. This simplifies everything
by allowing you connect before creating the RPC client.
BREAKING CHANGE: `Endpoint::channel` was removed in favor of
an async `Endpoint::connect`.
This removes custom content-types in favor of
just using `application/grpc`. There is some
confusion around the specification but most
grpc implementations ignore the `+` and
anything after.
